(1) Field of the Invention
The present invention relates to a method for manufacturing carbon fiber by infusibilization of pitch fiber followed by carbonization or graphitization.
(2) Description of the Related Art
Carbon fiber was originally manufactured using rayon as the precursor material. Due to their characteristics and economy, however, PAN-type carbon fiber using polyacrylonitrile as the precursor material or pitch-based carbon fiber using petroleum pitch as the precursor material have become dominant.
The technology for manufacturing so-called high-performance pitch-based carbon fiber with a high strength and high Young's modulus using coal or petroleum pitch has been winning wide interest due to its superior economy. For example, the method of manufacturing a high strength, high modulus carbon fiber by infusibilization of pitch fiber obtained by melt spinning mesophase or premesophase pitch in an oxidizing atmosphere followed by carbonization in an inert atmosphere is known (Japanese Unexamined Patent Publication (Kokai) No. 49-19127, Japanese Unexamined Patent Publication (Kokai) No. 51-11983, and Japanese Unexamined Patent Publication (Kokai) No. 59-53717).
The method for manufacturing pitch-based carbon fiber generally comprises a process for the preparation of the pitch, a process for the melt spinning of the pitch, a process for infusibilization, where the spun pitch fiber is heated for a long period in an oxidizing atmosphere (for example, heated at 250.degree. C. to 500.degree. C. for approximately 1 to 2 hours) for infusibilization, and a carbonization process, where the infusibilized fiber is further heated at a high temperature for carbonization or graphitization for the formation of carbon fiber.
Of these, the infusibilization process is an extremely important process in the sense of governing the productivity and fiber physical properties in the industrial manufacture of pitch-based carbon fiber. In particular, the reduction of the infusibilization time is important for the improvement of the productivity of carbon fiber. For this reason, the temperature, rate of temperature rise, atmosphere gas, and the like of the infusibilization are being studied. Further studies are also being conducted on various infusibilization accelerators.
For example, Japanese Unexamined Patent Publication (Kokai) No. 49-75828 corresponding to DE 2,350,769, Japanese Unexamined Patent Publication (Kokai) No. 51-75126 corresponding to GB 1,454,629, Japanese Unexamined Patent Publication (Kokai) No. 59-1723, etc. propose methods for the treatment of pitch fiber under an oxidizing gas atmosphere containing a halogen, particularly chlorine. However, these methods suffer from the problems of insufficient reduction of the infusibilization time, insufficient strength of the obtained fiber, etc. Further, when using chlorine, the chlorine is used in a state mixed with the oxidizing gas and at a high temperature, so there are problems in the work environment and corrosion of facilities, making such use unpreferable. Further, Japanese Unexamined Patent Publication (Kokai) No. 51-88729 corresponding to GB 1,538,042 proposes a method for steeping the pitch fiber in an aqueous solution of chlorine and then heat treating it in an oxidizing atmosphere. This method, however, suffers from the same problems as mentioned above.
In addition, proposals have been made for the use of metal salts, ammonium salts, inorganic acids, nitrides, and the like as accelerators for infusibilization. From the viewpoint of the effects of acceleration of infusibilization and the physical properties of the carbon fiber after carbonization, however, no satisfactory one has yet been found.
The present inventors and other previously made studies on the treatment temperature, rate of temperature rise, and the like in the infusibilization process, and as a result, selected a specific range of these conditions, proposing a method where the time required for infusibilization could be reduced to less than 30 minutes (Japanese Patent Application No. 59-9455). A further reduction of the time of the infusibilization process, however, is desired.